This section is from the book "Complete Self-Instructing Library Of Practical Photography", by J. B. Schriever. Also available from Amazon: Complete Self-Instructing Library Of Practical Photography.
Why The Margins Are Not Sharp. As we have previously indicated, the two defects in the rectilinear lens which render the marginal definition imperfect are astigmatism and curvature of field. Astigmatism is an error existing in oblique rays of light, and so effects, to any noticeable extent, the margins only. To study it you may fix against the window a large sheet of thin cardboard or stout black paper - ordinary brown paper has too many holes in it for this purpose. Now make a pin-hole with a good size pin, as smoothly as possible. Place the camera about a yard from the sheet of cardboard, and, using the rectilinear lens, focus the pin-hole sharply on the center of the ground-glass. Then rotate the camera until the image falls on the margin of the ground-glass, and rocking it backwards and forwards you will find that instead of a sharp image you secure, first, a long narrow ellipse - almost a line in fact - running vertically; next, a circular blur, and then a long ellipse, this time running horizontally.
655. Now turn the camera around, take down the sheet of thin card, and fix up, about a yard away from the lens, in a good light, a white card, on which you have drawn, in good black ink, a plain cross like the + sign. The arms may be about an inch long, and should be as thick as an ordinary stroke of a pin. Regard this cross as consisting of a great number of points like the original pin-hole. With the screen in the first position each point is represented as the long ellipse running vertically. The upright arm of the cross is thus clearly defined, because the many ellipses overlap each other, but the horizontal arm is represented by multitudes of ellipses arranged side by side instead of end to end, and is thus blurred. If the lines of the cross are very fine, it is possible that one arm may disappear entirely on the ground-glass image.
656. Moving the screen to the second position we get both arms equally blurred, and still further in the third position the ellipses running in horizontal directions give us an image with the horizontal arm sharply defined and the vertical arm blurred and almost invisible. Obviously, what happens in the case of the cross, happens to the tree twigs in your landscape with lines and architecture, and so on with all other subjects.
Curvature Of Field. A further reason for the blurring of the margins is found in the fact that the image formed by the rapid rectilinear lens is not on a flat plane, but is hollow like a saucer - the field is said to be curved. The optician can eliminate astigmatism, but as he does the field becomes more curved. He can also flatten the field, but to do so increases the astigmatic error. Aided by the comparatively new Jena glasses he can construct a lens which has both a flat field and freedom from astigmatism. These glasses are more costly, and the degree of accuracy in polishing their surfaces needs to be greater if errors are to be satisfactorily eliminated, hence the greater the cost of the modern lens.
658. The use of a smaller stop, as you have seen, diminishes, to a certain degree both these errors, but at the same time lessens the tendency of rapidity in the lens.